Organic producers must “maintain or improve the physical, chemical, and biological condition of the soil and minimize erosion.” The application of raw manure to organic crops and the process of making of composts from animal manures are both regulated under organic production.
Healthy soil is the basis for organic agriculture. Healthy soil can provide an abundant crop of healthy plants, which in turn provide healthy food and feed. When grown in good soils, crops are better able to resist disease, survive drought, and tolerate insects.
Green manures are crops grown specifically for soil improvement. They are typically incorporated into the soil after they have produced a large amount of biomass, or fixed a significant amount of nitrogen in the case of legumes. Managing green manure crops to increase organic matter and provide the maximum amount of nitrogen to the following crop is both an art and a science
Manures from conventional systems are allowed in organic production, including manure from livestock grown in confinement and from those that have been fed genetically engineered feeds. Manure sources containing excessive levels of pesticides, heavy metals, or other contaminants may be prohibited from use. Such contamination is likely present in manure obtained from industrial-scale feedlots and other confinement facilities. Certifiers may require testing for these contaminants if there is reason to suspect a problem. If a manure source is suspected of being contaminated with excessive amounts of prohibited substances, appropriate testing should be conducted. If test results indicate that the manure is free of excessive contamination, and it is subsequently used in production, the test results should be kept on file.
The 90–120-Day Rule
Application of manure to organic crops is restricted by what is known as the 90–120-day rule, as described in § 205.203(c)(1). You may not apply raw, uncomposted livestock manure to food crops unless it is:
- Incorporated into the soil a minimum of 120 days prior to harvest when the edible portion of the crop has soil contact; OR
- Incorporated into the soil a minimum of 90 days prior to harvest of all other food crops.
Incorporation is generally assumed to mean mechanical tillage to mix the manure into the soil. Crops that have soil contact include leafy greens, melons, squash, peas, and many other vegetables. Any harvestable portion of a crop that can be splashed with soil during precipitation or irrigation might be considered to have soil contact. Crops that do not have soil contact include tree fruits and sweet corn.
Take note that the 90- and 120-day restrictions apply only to food crops; they do not apply to fiber crops, cover crops, or to crops used as livestock feed.
The regulations define compost as “the product of a managed process through which microorganisms break down plant and animal materials into more available forms suitable for application to the soil…” Compost used in organic production must be made according to the criteria set out in § 205.203(c)(2). This section of the regulations specifies that:
- The initial carbon:nitrogen ratio of the blended feedstocks must be between 25:1 and 40:1.
- The temperature must remain between 131 °F and 170 °F for 3 days when an in-vessel or a static-aerated-pile system is used.
- The temperature must remain between 131 and 170°F for 15 days when a windrow composting system is used, during which period the windrow must be turned at least five times.
The composting procedures above are adapted from U.S. Environmental Protection Agency (EPA) and USDA’s Natural Resources Conservation Service (NRCS) guidelines for composting biosolids. This policy was established to ensure the elimination of pathogens that cause illness in humans.
Some organic farmers apply compost teas to crops or soil to increase the populations of beneficial microbes. If compost tea will be applied to organic crops, it is critical that the compost used to produce the extract has been made according to USDA organic regulations. The procedures for making both the compost and the compost tea must be explained in your Organic Systems Plan. Applications of teas made from uncomposted manure must follow the 90-120- day rule.
Vermicompost is compost that uses worms to digest the feedstocks. Since feedstocks may include animal manures, there has been debate as to whether the 90-120-day rule should apply to vermicompost. The NOP has issued the following guidance: feedstocks for vermicompost materials may include organic matter of plant or animal origin. Feedstocks should be thoroughly macerated and mixed before processing.
Processed Animal Manures
Heat-treated, processed manure products may be used in organic production. There is no required interval between application of processed manure and crop harvest. From the standpoint of the farmer, of course, these inputs would be applied well before harvest, so that the nutrients would be available to the crop. To be considered processed, the manure must be heated to 150 °F for 1 hour and dried to 12 percent moisture or less.